Physiological studies on supercooled killifish (<i>Fundulus heteroclitus</i>) I. Serum inorganic constituents in relation to osmotic and ionic regulation at subzero temperatures

Abstract

Abstract Physiochemical properties (serum osmolality and blood pH), serum inorganic ions (sodium, potassium, calcium, magnesium, chloride, phosphate and bicarbonate) and tissue water were studied in parallel groups of adult male Fundulus heteroclitus acclimated to various temperatures (30°C, 20°C, 10°C, 4°C, 2°C, −1°C and −1.5°C) in salt water under otherwise constant laboratory conditions. When the acclimation temperature was lowered from 20°C to −1.5°C, serum osmolality increased by 20%. This increase was not indicative of osmoregulatory failure, however, since the proportion of total serum osmolality accounted for by inorganic electrolytes dropped from 98% at 20°C to 93% at −1.5°C. Serum electrolytes such as sodium, chloride, calcium, magnesium and bicarbonate increased in the subzero cold by 12%, 17%, 30%, 33% and 11% respectively whereas serum potassium and inorganic phosphate levels were unchanged. Blood pH was significantly higher at 10°C than at any other temperature. The water content of liver, testes and muscle decreased by 8% in the subzero cold. These changes in serum electrolytes and tissue water did not indicate osmoregulatory failure, however, since the new levels, once established, were maintained as long as the fish were alive. Although osmotic and ionic regulation were not as effective in the cold, they were by no means so poor as to cause death by osmotic imbalance.